(i) Field of the Invention
The present invention relates to a power amplification device which is applied to reduce the operating current of an output amplifier.
(ii) Description of the Related Art
As an example of the conventional art in which the efficiency of an amplifier is raised by setting the midpoint voltage of the amplifier very close to an earth level, instead of setting it to half of the supply voltage, a power amplification device is described in Japanese Patent Application Laid-open No. 338738/1994. Such a power amplification device is constituted as shown in FIG. 5.
In FIG. 5, an input signal is applied to a negative input terminal of a first differential amplifier 1, and output signals reversed in phase with respect to each other are generated from positive and negative output terminals of the first differential amplifier 1. The positive and negative output signals of the first differential amplifier 1 are amplified by first and second output amplifiers 2 and 3. The first and second output amplifiers 2 and 3 form BTL amplifiers, and a load RL is BTL driven by output signals of the first and second output amplifiers 2 and 3.
Moreover, the output signals of the first and second output amplifiers 2 and 3 are non-linearly added by a non-linear adder circuit 4. The non-linear adder circuit 4 operates as an adder circuit when output signal levels of the first and second output amplifiers 2 and 3 are less than predetermined levels, while it operates as a clamp circuit when the output signal levels are more than the predetermined levels. An output signal of the non-linear adder circuit 4 is applied to a negative input terminal of a second differential amplifier 5. An output signal corresponding to a difference of positive input terminal from reference voltage Vr is applied to a common terminal C of the first differential amplifier 1. The common terminal C determines output direct current voltages of the first and second output amplifiers 2 and 3, and the output direct current voltages are controlled in response to the output signals of the first and second output amplifiers 2 and 3. Thereby, the output direct current voltages of the first and second output amplifiers 2 and 3 are set to voltages close to earth levels. Output signals a and b of the first and second output amplifiers 2 and 3 form half-wave output signals as shown in FIGS. 3A and 3B.
Furthermore, the output signals a and b of the first and second output amplifiers 2 and 3 are added by an adder circuit 6. An output signal c of adder circuit 6 is applied to a switching power supply 7. By performing switching in accordance with the level of the output signal of the adder circuit 6, a supply voltage Vx is generated following the output signals of the first and second output amplifiers 2 and 3. Specifically, as shown in FIG. 3C, the supply voltage Vx from the switching power supply 7 is analogous to the output signals a and b of the first and second output amplifiers 2 and 3. Subsequently, the supply voltage Vx is applied to the first and second output amplifiers.
In the power amplification device of FIG. 5, since the supply voltage Vx is changed in accordance with changes in output levels of the first and second output amplifiers 2 and 3, power saving can be realized. However, since the first and second output amplifiers 2 and 3 use constant currents from constant-current sources as operating currents, it is difficult to save much power even by changing the supply voltage Vx.